Fuel cell in which a fluid circulates essentially parallel to the electrolytic membrane and method for production of such a fuel cell

ABSTRACT

The invention relates to a fuel cell, comprising a substrate supporting an electrolytic membrane with first and second faces on which first and second electrodes are respectively arranged. The first and second electrodes respectively comprise first and second catalytic elements and first and second fluids are respectively provided for supply to the neighbourhood of the first and second catalytic elements. The supply of the first fluid on the neighbourhood of the first catalytic element is embodied such as to generate a circulation essentially parallel to the first face of the electrolytic membrane, in a cavity formed in the substrate.

BACKGROUND OF THE INVENTION

The invention relates to a fuel cell, and more particularly a micro fuelcell, comprising a substrate supporting an electrolytic membranecomprising first and second faces on which first and second electrodesare respectively arranged, the first and second electrodes respectivelycomprising first and second catalytic elements, circulation means beingdesigned to bring first and second fluids respectively in proximity tothe first and second catalytic elements.

The invention also relates to a method for production of such a fuelcell.

STATE OF THE ART

In fuel cells, providing the electrodes with reactive fluid and removingthe products formed when the cell operates represent two majordifficulties, in particular in micro fuel cells used in portableequipment. Miniaturization of fuel cells does in fact impose storage anda circulation circuit for the fuel, the combustive-fuel and the productsformed in the course of operation of the cell, in very small volumes.

The fuels used in microcells are generally in liquid form. As liquidfuels have a higher energy volume density than that of hydrogen, theyoccupy a smaller volume than hydrogen. Thus, it is commonplace to usefuel cells using methanol as fuel, these cells being better known underthe name of DMFC (Direct Methanol Fuel Cells). The methanol is oxidizedat the anode, on an active catalytic layer, to give protons, electronsand carbon dioxide. A proton conducting membrane arranged between theanode and a cathode conducts the protons to the cathode so as to makethe protons react with oxygen and form water. The carbon dioxide andwater forming respectively at the anode and the cathode when the celloperates therefore have to be removed.

In general manner, it is known to use supply circuits of the electrodesand of the electrolytic membrane. The circuits are generally in the formof supply channels and/or microporous diffusion layers performing supplyof the fluids perpendicularly to the electrodes or to the membrane.

Thus, the document FR-A-2,814,857 describes a micro fuel cell comprisingan oxygen electrode and a fuel electrode, the fuel preferably beingformed by a mixture of methanol and water. A microporous supportimpregnated with an electrolytic polymer forming an electrolyticmembrane is arranged between the two electrodes. The microporous supportis formed by an oxidized semi-conducting material made porous to formchannels parallel to one another. The channels enables electrochemicalexchanges to be made between the anode and cathode. The microporoussupport is supplied with fuel and with combustive-fuel by diffusionchannels respectively connected to a fuel source and an air source.

It is also known to use a porous diffusion layer to supply an electrodewith reactive fluid, as represented in FIG. 1. Thus, a fuel cell 1comprises a substrate 2 supporting an anode 3, an electrolytic membrane4 and a cathode 5. An anodic current collector 6 is arranged on theanode 3 and circulation of the fuel is tangential to the anode 3. Airsupply to the cathode is performed by means of circulation channels 7formed vertically in the substrate. The circulation channels 7 thereforeenable air to be transported from an air source (not represented) to amicroporous diffusion layer 8 arranged between the cathode 5 and acurrent collector 9. A fuel cell of this kind has been described in thedocument WO-A-0,045,457. The fuel cell thus comprises a substratesupporting first and second electrodes between which an electrolyticmembrane is arranged. Supply of the first electrode with reactive fluidis performed by a porous thin layer arranged between the first electrodeand a substrate. Said substrate comprises vertical diffusion channelsconnected to a cavity itself supplied by a fuel source. This type ofreactive fluid supply is however not satisfactory. The residual fluidssuch as water forming at the cathode in the fuel cell 1 are in factremoved by the same circulation channels as the reactive fluid such asthe air in the fuel cell 1, in the opposite direction. Making twoopposite flows circulate in a circulation channel having a relativelysmall diameter limits the access of the reactive fluids to the cathode.

OBJECT OF THE INVENTION

It is an object of the invention to remedy these shortcomings and moreparticularly to propose a fuel cell enabling both efficient and quickremoval of the compounds formed when operating and enabling the reactivefluids to be quickly renewed.

According to the invention, this object is achieved by the fact that thecirculation means of the first fluid are designed in such a way as tomake the latter flow in a direction substantially parallel to the firstface of the electrolytic membrane, in a cavity formed in the substrate.

According to a development of the invention, the cavity comprises aplurality of studs supporting said electrolytic membrane.

According to a preferred embodiment, the first catalytic element isformed by a plurality of catalytic zones respectively arranged at thetop of the studs of the cavity.

According to another preferred embodiment, the first catalytic elementis formed by a plurality of catalytic zones, said catalytic zones beingrespectively formed by the studs.

It is a further object of the invention to provide a method forproduction of such a fuel cell that is easy to implement and usingtechniques implemented in the microtechnology field.

According to the invention, this object is achieved by the fact that themethod for production consists in performing reactive ion etching in thesubstrate so as to form the cavity and the plurality of studs at thesame time.

According to a development of the invention, the method for productionconsists in depositing on the top of each stud, by physical vapourdeposition, a growth promoting substance designed to foster formation ofa catalyzer support whereon a catalytic layer is deposited byelectroplating.

According to the invention, this object is also achieved by the factthat the method for production consists in etching the cavity in thesubstrate and in then forming the plurality of studs by electrolyticgrowth.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIG. 1 represents in cross-section a fuel cell of the prior art.

FIG. 2 is a cross-sectional view of a particular embodiment of a fuelcell according to the invention.

FIG. 3 represents an overall view of a part of the fuel cell accordingto FIG. 2.

FIG. 4 represents a top view of a cavity of a fuel cell according to theinvention.

FIG. 5 represents a top view of the circulation means of a fluid in thefuel cell according to FIG. 1.

FIGS. 6 to 8 illustrate different steps of a first method for productionof the catalytic zones in the fuel cell according to FIG. 3.

FIGS. 9 to 14 illustrate different steps of a second method forproduction of a fuel cell according to the invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

A fuel cell according to the invention comprises a substrate supportingan electrolytic membrane comprising first and second faces. First andsecond electrodes are respectively arranged on the first and secondfaces of the electrolytic membrane and they respectively comprise firstand second catalytic elements designed to trigger an electrochemicalreaction. First and second fluids are respectively designed to bebrought near to the first and second catalytic elements. Supply of thefirst fluid is thus performed in such a way as to make the latter flowsubstantially parallel to the first face of the electrolytic membrane ina cavity formed in the substrate, and to bring it into contact with thefirst catalytic element. The first fluid associated with the firstcatalytic element can thus be either the combustive-fuel associated withthe catalytic element of the cathode or the fuel associated with thecatalytic element of the anode. The cavity formed in the substrate cancomprise a plurality of studs supporting the electrolytic membrane.

In a particular embodiment represented in FIGS. 2 and 3, a cavity 10 isformed in the substrate 2 of a fuel cell 1 and it comprises a pluralityof studs 11. The cavity 10 is designed to bring a first fluid to theproximity of a first electrode and the studs 11 preferably form anetwork designed to distribute the first fluid homogeneously in thecavity 10. For example, in FIG. 2, the first fluid is a combustiblefluid such as a mixture of water and methanol and the first electrode isan anode. Inlet of the combustible fluid to the cavity 10 and outletthereof from the cavity 10 can be performed by any type of suitablemeans. For example, the walls of the cavity 10 can be porous or they cancomprise inlet and outlet apertures connected to circulation channels orto a fuel source. Thus, the flow of combustible fluid generated in thecavity 10 and represented by an arrow 12 in FIG. 2 moves horizontally inthe cavity 10 between the studs 11 and substantially parallel to thefirst face 4 a of the electrolytic membrane 4.

The studs 11 can be of any suitable shape. They can for example have acircular, rectangular or polygonal cross-section. They can also bedistributed in the cavity 10 in any kind of arrangement, the studs 11being able for example to be aligned in several rows or form azig-zagged network. This arrangement is adjusted so that the combustiblefluid can be distributed homogeneously in the cavity 10. The number ofstuds 11 in the cavity 10 can also be adjusted according to the time thecombustible fluid is to spend in the cavity 10. The fuel cell can alsocomprise means for controlling the combustible fluid flow, so as toadjust the flow time of the combustible fluid in the cavity andtherefore the electrochemical reaction time.

The studs 11 preferably have the same dimensions and their height isequal to the depth of the cavity 10. For example, the height of thestuds can be 30 micrometers and their diameter can be comprised between10 micrometers and 40 micrometers for cylindrical studs. In addition,the distance between two studs is preferably less than or equal to 50micrometers, so that all of the studs 11 can support an electrolyticmembrane 4.

The electrolytic membrane 4 comprises first and second faces 4 a and 4b, respectively designed to be in contact with the first and secondcatalytic elements of the first and second electrodes. Thus, the firstface 4 a of the electrolytic membrane 4 is placed on the studs 11 andthe ends of the electrolytic membrane 4 are securedly fixed to thesubstrate 2. The second face 4 b of the electrolytic membrane 4 iscovered by a catalytic element 13 in the form of a thin film and adiscontinuous current collector element 14, the catalytic element 13 andthe current collector element 14 thus forming the second electrode. Thefluid associated with the second electrode is, in FIG. 2, a combustivefluid such as air and the second electrode corresponds to the cathode ofthe fuel cell. The combustive fluid flow is schematized in FIG. 2 by anarrow 15 located above the cathode. Thus, the air flows parallel to thecathode so that the air flow can remove the water produced at thecathode to the outside of the fuel cell (arrow 16) when the fuel cell isoperating.

On the top of each stud 11, there is preferably arranged a catalyticzone 17 designed to trigger an electrochemical reaction with thecombustible fluid. The set of catalytic zones 17 thus forms thecatalytic element of the anode. As the studs 11 support the electrolyticmembrane 4, each catalytic zone 17 is in contact with the first face 4 aof the electrolytic membrane 4 and a current collector 18 is depositedon the surface of the studs 11 and on the walls of the cavity 10.

Such a fuel cell enables the combustible fluid to be made to flowsubstantially parallel to the first face 4 a of the electrolyticmembrane (FIG. 3). The flow thus created enables the combustible fluidto be renewed at the level of the catalytic zones 17 of the anode.Moreover, unlike a circulation circuit according to the prior art (FIGS.1 and 5), the products formed at the anode when the fuel cell operatesare driven by the flow of combustible fluid. In this way, the productsformed do not slow down renewal of combustible fluid to the catalyticzones 17.

Indeed, in the fuel cell 1 according to FIG. 2, the flow of combustiblefluid, represented by the arrow 12 in FIG. 4, circulates between thestuds 11 of the cavity 10 and drives with it the residual fluids formedat the anode, such as carbon dioxide for a combustible fluid comprisingmethanol and water. In a fuel cell according to the prior art on theother hand, the flow of combustible fluid and the flow of residualfluids, respectively represented by the arrows 19 and 20 of FIG. 5,circulate in opposite directions in the same circulation channels 21.The circulation channels 21 are formed in the substrate 2 and transportthe flow of combustible fluid perpendicularly to the electrolyticmembrane.

According to a particular embodiment of fabrication of the fuel cell 1,reactive ionic etching (RIE) in the substrate 2 enables the cavity 10and studs 11 to be formed simultaneously. The substrate can be made ofsilicon, ceramic or plastic. Once the cavity 10 and studs 11 have beenformed, physical vapour deposition of platinum is performed on thesurface of the studs 11 and on the walls of the cavity 10 so as to forma thin film having a thickness of about one micrometer and forming thecurrent collector 18 of the anode.

The catalytic zones 17 are then made at the top of the studs 11, asrepresented in FIGS. 6 to 8. Thus, a layer of protective resin 22 isdeposited in the cavity 10 up to a predetermined height so that the toppart of the studs 11 is free. Physical vapour deposition of a growthpromoting substance 23 is performed in the cavity 10 so as to cover thetop part of the studs 11 with protective resin (FIG. 6). After the layerof protective resin 22 has been removed (FIG. 7), only the top parts ofthe studs 11 are covered with a layer of growth promoting substance 23designed to foster formation of a catalyzer support 24 at the top ofeach stud 11. The catalyzer support 24, preferably formed by carbonnanotubes, is then covered with a catalytic active layer 25, byelectroplating (FIG. 8). The catalyzer support 24 and the catalyticactive layer 25 form a catalytic zone 17 of the catalytic element of theanode.

Once the catalytic zones 17 have been formed, the electrolytic membrane4, preferably made of Nafion®, is spread by a centrifugation process,also called spin coating, and is then dried. The small space between twostuds 11 enables a volume of air to be trapped preventing the stillliquid material of the membrane from running before it has dried. Thecatalytic element of the cathode, preferably formed by a mixture ofplatinum-plated carbon and Nafion®, is then spread by sputtering on thedried electrolytic membrane 4, then the current collector 14 of thecathode is deposited by physical vapour deposition.

According to an alternative embodiment, the catalytic zones 17 of thecatalytic element of the anode can be respectively formed by the studs11 of the cavity 10. The cavity and studs are then formed successively.Thus, as represented in FIGS. 9 to 14, several fuel cells can be made onthe same substrate. Two cavities 10 are etched in the substrate 2 andtheir walls are metallized (FIG. 9). The studs 11 are then formed byelectrolytic growth and a layer of thick resin 26 is deposited in thecavities 10 (FIG. 10). Spaces 27 corresponding to the required positionfor the studs 11 are created, by lithography, in the resin layer 26(FIG. 11). Then the studs 11 are formed in the spaces 27 by electrolyticgrowth of platinum (FIG. 12). The studs 11 preferably comprise, at thetop part thereof, a broader zone constituting a head 28. The layer ofthick resin 26 is then removed to free the cavities 10 (FIG. 14). Alayer designed to form electrolytic membranes 4, preferably made ofNafion®, is deposited above the cavities 10 so that the electrolyticmembranes 4 are supported by the studs 11. The catalytic element and thecurrent collector of the cathode are then deposited on the electrolyticmembrane by means of any type of known technique.

The invention is not limited to the embodiments described above. Thus,the fluid designed to flow substantially parallel to the first face ofthe electrolytic membrane in the cavity can be the combustive fluid.Likewise, the catalytic element designed to be in contact with saidfluid can be continuous. For example, the catalytic zones constituted bythe studs or formed at the top of the studs can be joined so as toobtain a continuous catalytic element. The combustible fluids can be ofany type, liquid or gaseous. The fuel cell can more particularly be ofthe DMFC type and it can also be a micro fuel cell of the same type asthose used in portable equipment.

1-14. (canceled)
 15. Fuel cell comprising a substrate supporting anelectrolytic membrane comprising first and second faces on which firstand second electrodes are respectively arranged, the first and secondelectrodes respectively comprising first and second catalytic elements,circulation means being designed to bring first and second fluidsrespectively in proximity to the first and second catalytic elements,wherein the circulation means of the first fluid are designed in such away as to make the latter flow in a direction substantially parallel tothe first face of the electrolytic membrane, in a cavity formed in thesubstrate and comprising a plurality of studs supporting saidelectrolytic membrane.
 16. Fuel cell according to claim 15, wherein thedistance between two studs is less than or equal to 50 micrometers. 17.Fuel cell according to claim 15, wherein the first catalytic element isformed by a plurality of catalytic zones respectively arranged at thetop of the studs of the cavity.
 18. Fuel cell according to claim 15,wherein the first catalytic element is formed by a plurality ofcatalytic zones, said catalytic zones being respectively formed by thestuds.
 19. Fuel cell according to claim 18, wherein the studs comprise,at the top part thereof, a broader zone forming a head.
 20. Fuel cellaccording to claim 15, wherein the studs have a circular cross-section.21. Fuel cell according to claim 15, wherein the studs have arectangular cross-section.
 22. Fuel cell according to claim 15, whereinthe studs have a polygonal cross-section.
 23. Fuel cell according toclaim 15, wherein the studs form a network designed to distribute thefirst fluid homogeneously in the cavity.
 24. Fuel cell according toclaim 23, wherein the network is arranged in zig-zagged manner. 25.Method for production of a fuel cell according to claim 15, consistingin performing reactive ionic etching in the substrate so as tosimultaneously form the cavity and the plurality of studs.
 26. Methodfor production according to claim 25, consisting in depositing on thetop of each stud, by physical vapour deposition, a growth promotingsubstance designed to foster formation of a catalyzer support whereon acatalytic layer is deposited by electroplating.
 27. Method forproduction according to claim 26, wherein the catalyzer support isformed by carbon nanotubes.
 28. Method for production of a fuel cellaccording to claim 15, consisting in etching the cavity in the substrateand in then forming the plurality of studs by electrolytic growth.